A pressure produced by hydraulic equipment has been increased every year, so that troubles such as scuffing of a hydraulic pump have frequently been caused. Therefore, ISO standards have required an FZG scuffing test. FZG scuffing resistance is usually improved by adding an acid phosphate or an amine salt thereof or, alternatively, an active sulfur compound (see, for instance, Patent Literature 1). Patent Literature 1 suggests a lubricating oil composition prepared by blending a base oil with (a) phosphate in an amount of 0.01 to 5 mass %, (b) an amine salt of an acid phosphate in an amount of 0.005 to 1 mass % and (c) a sulfuric extreme pressure agent in an amount of 0.01 to 1 mass %. However, since an oil temperature is raised with an increased pressure produced by hydraulic equipment, these additives may unfavorably turn to sludge. The sludge causes filter clogging, valve lockout and pump wear. Further, an amine salt of an acid phosphate and an alkenyl succinate, which is used as a general rust inhibitor, tend to lower the pH of eluted water, so that polyurethane rubber is likely to deteriorate under the presence of water.
A suggested composition that contains neither an amine salt of an acid phosphate nor an alkenyl succinate is, for instance, a hydraulic oil composition prepared by blending a base oil with (A) phosphate or thiophosphate in an amount of 150 to 3000 mass ppm in terms of a phosphorus content, (B) a sulfur compound having a dithio bond in a molecule in an amount of 0.02 to 0.5 mass % in terms of a sulfur content, (C) a metal sulfonate in an amount of 0.05 to 5 mass %, and (D) an overbased metal salicylate or an overbased metal phenate in an amount of 0.01 to 0.5 mass % (see Patent Literature 2).